Polycarbonates are well known, commercially important materials which are produced in large quantities. Such polymers are typically prepared by reacting a carbonate precursor with a dihydric phenol to provide a linear polymer consisting of units of the dihydric phenol linked to one another through carbonate linkages. These polymers have outstanding mechanical, thermal, and optical properties such as high tensile strength, optical clarity (transparency), thermal and dimensional stability and impact strength.
These aromatic polycarbonates differ from most thermoplastic polymers in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. Newtonian flow is defined as the type of flow occurring in a liquid system where the rate of shear is directly proportional to the shearing force. However, in contrast to most thermoplastic polymers, polycarbonates prepared from dihydric phenols exhibit Newtonian flow at normal processing temperatures and shear rates below 300 reciprocal seconds.
Two other characteristics of molten thermoplastic polymers are considered to be significant for molding operations: melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt from distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and indicates the ability of the melt to support a stress. Both of these characteristics are important in extrusion blow molding, particularly in fabrication by extrusion blow molding. Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to polymers thus allowing their use in blow molding fabrication. In the usual blow molding operation, a tube of a molten thermoplastic is extruded vertically downward into a mold, followed by the introduction of a gas, such as air, into the tube thus forcing the molten plastic to conform to the shape of the mold. The length of the tube and the quantity of material forming the tube are limiting factors in determining the size and wall thickness of the object that can be molded by this process. The fluidity of the melt obtained from bisphenol-A polycarbonate, or the lack of melt strength as well as the paucity of extrudate swelling, serve to limit blow molding applications to relatively small, thin walled parts. Temperatures must generally be carefully controlled to prevent the extruded tube from falling away before it attains the desired length and the mold is closed around it for blowing. Consequently, the Newtonian behavior of polycarbonate resin melts has severely restricted their use in the production of large hollow bodies by conventional extrusion blow molding operations as well as the production of various other shapes by profile extrusion methods.
Thermoplastic randomly branched polycarbonates exhibit unique properties of non-Newtonian flow, melt elasticity and melt strength which permit them to be used to obtain such articles as bottles which were not heretofore easily or readily produced with linear polycarbonates. The thermoplastic, randomly branched polycarbonates can be prepared by reacting the branching agent with a dihydric phenol and a carbonate precursor.